Fluorocarbon resins, in particular vinylidene fluoride resins, are well known for their outstanding mechanical properties within a broad range of temperature, excellent resistance to high temperature, organic solvents and to various chemically aggressive environments.
Thanks to their properties, they are commonly used for manufacturing articles by extrusion or injection molding, e.g. for producing pipes, tubes, fittings, films, coatings, cable sheathings, flexible pipes and the like.
To further improve mechanical properties of these polymers, it is well known to increase their molecular weight, or, in other words, to increase their melt viscosity. Thus, although fluoropolyer of high molar mass (and thus high melt viscosity) are preferable because of improved mechanical properties, processing these materials is more difficult. In particular, in extrusion process, they display rheology problems, accounting for increased energy consumption during extrusion and more severe extrusion conditions to be applied (with consequent risks of thermal degradation of the polymer). In this case, finished parts (extrusion or injection molded) made from these fluoropolyer have generally surface defects like cracks, shark-skin, fish-eyes and the like.
Processing aids have been thus largely used to obviate these problems; nevertheless, they have several drawbacks. Due to their limited thermal stability at processing temperature, benefits of their addition are lost and fumes are released during processing; thermal degradation residues generate in final parts structural defects and voids that can be detrimental to mechanical properties.
Perfluoropolymers have been thus employed to aid the extrusion processes and improve the final surface characteristics of the extrudate.
Thus,
Patent Citation 0001: U.S. Pat. No. 4,423,192 (UGINE KUHLMANN (FR)). 1983 Dec. 27.
discloses lubricated polyvinylidene fluoride compositions comprising from 0.1 to 10% by weight of tetrafluoroethylene homopolymers or copolymers, containing at least 90 mole percent of tetrafluoroethylene groups, of low molecular weight.
Patent Citation 0002: GB 1255081 (PENNWALT CORP (US)). 1971 Nov. 24.
discloses vinylidene fluoride polymer compositions comprising high-molecular weight tetrafluoroethylene homopolymers or normally solid high-molecular weight co-polymers of tetrafluoroethylene containing at least 90 mole percent of tetrafluoroethylene units.Patent Citation 0003: WO WO 2003050183 A (3M INNOVATIVE PROPERTIES). 2003 Jun. 19.discloses, inter alia, melt processable fluorothermoplastic compositions comprising a major amount of a first semi-crystalline fluorinated copolymer and a minor amount of a second fluoropolymer effective to reduce melt defects in the compositions.
Finally,
Patent Citation 0004: US 20030198769 (3M INNOVATIVE PROPERTIES). 2003 Oct. 23.
discloses fluoropolymer blends combinations comprising a partially-fluorinated thermoplastic polymer and a perfluorinated thermoplastic polymer.
Despite these attempts, there is always a great need for composition wherein a uniform dispersion of the perfluoropolymer acting as processing aid is achieved. When mixing the processing aid with the matrix in the composition of the prior art, phase-separated domains of large size and irregular shape are obtained: this uneven distribution prevents the perfluoropolymer to efficiently improve processing. Thus this approach fails to provide the targeted advantages, and, notwithstanding the addition of the processing aid, it is not possible to process the composition materials at higher output rates while maintaining satisfactory quality in final parts finish and keeping good mechanical properties.
This invention thus aims at providing a thermoplastic fluoropolymer composition in which the efficiency of the perfluoropolymer as processing aid in improving rheological behavior is increased and in which the mechanical properties are not negatively affected.